1. Field of the Invention
The present invention relates generally to electrochemical cells. More particularly, the present invention relates to a nonaqueous high pulse electrochemical cell having an anode comprising an alkali metal and a solid cathode. Preferably, the electrochemical cell of the present invention includes an alkali metal alloy anode such as lithium-aluminum anode active material, an anode current collector comprising nickel and a cathode comprising a calendared mixed metal oxide applied to a highly conductive metal screen. A preferred cathode material is silver vanadium oxide (Ag.sub.2 V.sub.4 O.sub.11) pressed onto a cathode current collector comprising aluminum. This electrochemical system is preferably activated with an electrolytic solution comprising an alkali-metal salt of hexafluorophosphate and more preferably lithium hexafluorophosphate dissolved in a mixture of organic solvents when the anode comprises lithium.
2. Prior Art
The prior art has described various nonaqueous alkali metal/solid cathode electrochemical cells. While many of the prior art cells describe one or more of the components of the present high pulse power cell, none of them describes the combination of an alkali metal anode, an anode current collector comprising nickel, a calendared mixed metal oxide cathode active material such as silver vanadium oxide applied to a cathode current collector comprising aluminum and an electrolytic solution comprising an ion-forming alkali metal of hexafluorophosphate wherein the ion-forming alkali metal is similar to that comprising the anode. It is this cooperative electrochemical system that results in a synergistic improvement in the discharge efficiency of the high pulse power cell of the present invention, which magnitude of discharge efficiency is not provided by any of the prior art cells.
Among the prior art cells describing various components of the present high pulse power cell, there is U.S. Pat. No. 4,830,940 to Keister et al., which is assigned to the assignee of the present invention. Keister et al. describes a nonaqueous lithium electrochemical cell having a mixed metal oxide cathode associated with a cathode conductor formed from a thin sheet of metal screen, for example titanium or stainless steel screen. The cathode can comprise silver vanadium oxide as the active material combined with a binder such as polytetrafluoroethylene, and additionally may include conductive additives such as graphite powder and acetylene black. The cathode assembly is formed by pressing the cathode active material onto the titanium or stainless steel screen and enclosing the assembly in an envelope of separator material such as polypropylene. The anode comprises an alkali metal, preferably lithium or a lithium alloy enclosed within the separator and suitable electrolytes for activating the cell include a lithium salt dissolved in an organic solvent, preferably 1M lithium hexafluoroarsenate (LiAsF.sub.6) dissolved in a 50:50 by volume mixture of propylene carbonate and dimethoxyethane (PC:DME).
The electrochemical cell of the present invention comprises a calendared cathode active material pressed onto an aluminum cathode current collector. This combination results in improved cell performance.
U.S. Pat. No. 5,154,992 to Berberick et al. describes a nonaqueous lithium/manganese dioxide electrochemical cell. Other suitable cathode materials include silver vanadium oxide mixed with carbon and polytetrafluoroethylene that has been pressed to form a porous solid structure. The separator is microporous polypropylene bonded to a non-woven polypropylene substrate and the nonaqueous electrolyte includes salts such as LiPF.sub.6 and LiAsF.sub.6. This patent does not describe calendaring of the cathode active material mixed with the binder and conductive additives and pressed onto an aluminum current collector to form the cathode component.
U.S. Pat. No. 5,114,811 to Ebel et al., which is assigned to the assignee of the present invention, describes an electrochemical cell having a lithium-aluminum alloy anode and a solid cathode. The cathode comprises silver vanadium oxide associated with a current collector that can comprise titanium and stainless steel. Again, this patent does not describe the cathode as being formed in a calendaring step and aluminum is not included as one of the materials that are useful for the cathode current collector.
U.S. Pat. No. 5,244,757 to Takami et al. describes a lithium secondary electrochemical cell having a carbonaceous negative electrode and a positive electrode comprising a lithium/metal oxide or lithium/mixed metal oxide material, an organic binder material and a conductive additive. Examples of suitable materials for the positive electrode include such compounds as manganese dioxide, a lithium-manganese composite oxide, a lithium-nickel oxide, a lithium-manganese-cobalt oxide, a lithium containing noncrystalline vanadium pentoxide, or chalcogen compounds such as titanium disulfate or molybdenum disulfate. Also, a part of Co in the lithium-cobalt oxide may be substituted with the metals such as transition metals, Sn, Al, Mg, T, V. The positive electrode is formed by combining these ingredients, kneading the mixture into a sheet and pressing the sheet against a current collector made of a material such as aluminum. This electrochemical system is activated with a lithium ion conductive solution that can include LiBF.sub.4, LiPF.sub.6 or LiCF.sub.3 SO.sub.3. However, silver vanadium oxide is not disclosed as a suitable mixed metal oxide, and in that respect, Takami et al. does not disclose the combination of an alkali metal anode, a cathode comprising a calendared silver vanadium oxide associated with an aluminum current collector and an activating electrolytic solution comprising lithium hexafluorophosphate provided as a primary electrochemical cell.
U.S. Pat. No. 4,980,250 to Takahishi et al. describes a secondary battery comprising a rechargeable positive electrode, a rechargeable lithium-containing negative electrode, a separator and an organic electrolytic solution. The positive electrode can comprise vanadium oxides combined with a conductive material such as graphite or acetylene black powder, and a binding agent such as polytetrafluoroethylene powder pressed into a film-like molded article. Suitable current collectors for both the anode and the cathode comprise metals such as nickel, titanium or stainless steel and the electrolyte is described as including LiAsF.sub.6 dissolved in a non-porotic, high-dielectric organic solvent.
U.S. Pat. No. 4,925,751 to Shackle et al. describes a secondary lithium cell having a cathode composition that preferably includes V.sub.6 O.sub.13 and electrically conductive carbon particles. This cell is activated with an ionically-conductive electrolyte comprising a single-phase solid solution of an ionizable alkaline metal salt, a solvent for the salt and a polymer which has been polymerized by exposure to actinic radiation, heat or which has been chemically polymerized. Although aluminum is listed along with stainless steel as suitable materials for the cathode current collector, the preferred current collector is described as a nickel foil having particles of nickel electrochemically deposited on the surface of the foil contacting the cathode composition. Further, the cathode active material is not calendared.
The high energy density electrochemical cell of the present invention is an improvement over the prior art alkali metal/mixed metal oxide cells, specifically lithium/silver vanadium oxide cells. In terms of current carrying capacity, energy density and ease of construction, among other advantages, the combination of an alkali metal-aluminum alloy anode associated with a nickel current collector, a calendared mixed metal oxide cathode active material pressed onto a cathode current collector comprising aluminum and a nonaqueous electrolytic solution comprising at least one ion-forming alkali metal salt of hexafluorophosphate, exhibits an electrochemically cooperative action that results in a synergistic improvement in cell discharge efficiency in comparison to the prior art cells. In particular, the electrochemical cell of the present invention discharges efficiently under pulse currents 1.6 times higher than those described by the prior art. In other words, the electrochemical cell of the present invention can be housed in a casing having a volume 40% to 50% smaller than the prior art cell systems while still being capable of discharging under equivalent current amplitudes. Additionally, the present alkali metal anode and electrolytic solution couple provides improvements in the voltage delay and discharge characteristics of the present high pulse power cell.
These and other improvements and advantages of the present invention will become increasingly more apparent to those skilled in the art by reference to the following descriptions and to the drawings.